This invention is directed to devices for determining resistance values, and, more particularly to devices for determining resistance values in the tens of megohm range and above (generally above 20 megohms).
In the past, many digital ohmmeters have been limited to directly measuring resistance values up to 20 megohms. Values above 20 megohms usually have been determined by measuring the combined resistance of a low resistor of known value connected in parallel with the high resistance of unknown value. Then a calculation is made, based on the measurement and the value of the known resistance, to determine the value of the unknown resistance. This procedure, while useful, is somewhat unsatisfactory. First, it is time consuming and subject to calculation error. In this regard, this technique only works well with resistances that are very high relative to the value of the known resistance. If the known resistance is not substantially lower than the value of the unknown resistance, the mathematical calculations become quite complicated. The present invention is directed to solving this problem and providing a device suitable for accurately, directly measuring resistance values above 20 megohms.
Prior attempts to solve this problem, i.e., to provide a device for directly measuring resistance values above 20 megohms, have usually involved adding an extended scale of one decade. This solution is generally unsatisfactory because the extended range has a very slow response time. Further, this solution only postpones the problem for one decade, i.e., resistance values above the extended range (200 megohms) still cannot be directly measured. Another approach has been to place an external power supply in series with the unknown resistance and a digital volt meter. This approach requires knowledge of the internal resistance of the volt meter, as well as knowledge of the exact value of the external voltage. In addition to being cumbersome, this technique is somewhat expensive to implement.
Therefore it is an object of this invention to provide a new and improved method of, and apparatus for, measuring resistance values in the tens of megohm range and above.
It is another object of this invention to provide an inexpensive and uncomplicated apparatus for determining resistance values in the tens of megohm range and above.
it is a still further object of this invention to provide an apparatus that provides an accurate, direct measurement of resistance values in the tens of megohm range and above.